Salts are often used to create solutions with specific pH values. Suppose you need to prepare a salt solution with a pH of about 4.5 and have available sodium dihydrogen phosphate, NaH₂PO₄, and sodium citrate, Na₂HC₆H₅O₇. Which should you use? Estimate the pH of

(a) 0.20 m NaH₂PO₄(aq);

(b) 0.20 m Na₂HC₆H₅O₇(aq). For citric acid, pK_a2 = 5.95 and pK_a3 = 6.39.

ANTICIPATE Both salts have acidic anions, so you should expect the pH to be less than 7 but greater than the value for a 0.20 m strong acid solution, which is 0.7.

PLAN Verify that S ≫ K_w/K_a2 and S ≫ K_a1; if these conditions are satisfied, you can use Eq. 1. You need the pK_as of the appropriate acids. For a diprotic species H₂A₂ (such as H₂PO₄^–), you need to consider the pK_as of H₃A (the pK_a1 of H₃A) and H₂A^2– (the pK_a2 of H₃A). For a monoprotic species HA (such as HC₆H₅O₇ ^2–), you need the pK_as of H₂A^– (the pK_a2 of H₃A) and HA^2– (the pK_a3 of H₃A). Use Eq. 1 in the form pH 5 ½(pK_a2 + pK_a3).